By Michael MacMillan
Radio modems. Two words that barely excite even telecommunication industry buffs. Add "on Mars" however, and suddenly everyone is interested.
Norman Pearl can verify it. Pearl is vice-president of engineering at DATARADIO, a Québec-based company responsible for the modem chip and firmware in Motorola's RNet 9600 radio modem Ñ the same unit that is helping the Mars Rover communicate with the landing craft.
"At parties, when people ask what you do . . . we can say we make the modems that go to Mars," he explained. "It's given our people a real thrill."
The Pathfinder project is the first of NASA's Discovery Program missions, and the second in the series of small, low-cost probes to investigate nearby planets. Pathfinder's telecommunication system consists of a two-way wireless UHF radio link between the lander and the small mobile rover (or "Sojourner") that operates much like a walkie-talkie system. Unfortunately, the system also resembles walkie-talkies in its limited range. Mission control personnel cannot speak directly to the Rover. All communications with the rover are done via the aid of the lander's communications interface.
Pearl was confused when he first received calls from Pathfinder engineers. Why would NASA be interested in outfitting an interplanetary probe with a common, relatively no-frills radio system? The answer was financing.
Experiencing a typical nineties cash-crunch, NASA wanted mission costs kept to a minimum. This meant engineers at the California-based Jet Propulsion Laboratory (JPL) faced the dilemma of sending a lightweight, mobile probe to Mars while operating on an unprecedented budget of around US $150 million (the actual cost was US $171 million). This was something the more senior staff, who recall the days of fat budgets, were not used to. To their credit, the team learned quickly.
"We came in far under budget on the Rover, and even gave some money back," said Scot Stride, a JPL engineer and member of the Pathfinder telecommunications and microwave team. "It takes more effort, but it also takes more common sense."
To accomplish this, JPL used off-the-shelf equipment wherever possible. Designing equipment in-house, a process that only ten years ago would have been considered a given, was now considered too risky. Scheduling was also critical. With only twenty-six months to get everything ready for the launch, JPL could hardly afford to waste time testing new equipment. In July 1992, JPL engineering personnel went window-shopping. The list of possible candidates was quickly narrowed to eleven modem and six antenna companies, from which the Motorola RNet 9600, was eventually chosen. At first their dealings were strictly with Motorola, but as their questions become more technical, they referred them to DATARADIO.
Pearl said JPL was definitely not a typical client. "They asked a lot of strange questions, like Ôhow do the units behave under conditions of cosmic radiation'," he recalled. Once JPL concluded that the RNet was the best candidate, they ordered thirty units. JPL based their decision on the RNet's relatively rugged design, small size and its low power consumption. Stride and his colleagues were also attracted to the radio's integrated circuitry and frequency. Although most JPL engineers initially favored the more "mechanically robust" military grade systems that would require very little rework for space flight, they are difficult to adjust, and much heavier than their civilian counterparts. When it comes to space missions, weight is money.
Opting for an off-the-shelf radio modem prompted concern. Pathfinder's radio had to withstand more than just the usual wear and tear. And DATARADIO hardly had flight in mind, let alone space flight, when they designed the RNet 9600. So JPL engineers went to work modifying the RNets, putting them through a series of tests, hoping to whittle thirty units down to the eight that they would need as flight and spare candidates.
DATARADIO worked closely with JPL during this period, making sure the adjustments were done with utmost precision. "One of our engineers spent a lot of hours on the phone and E-mail with those people," Pearl said. JPL replaced plastic connectors with more durable hardwire connections. Adjustments were also made to ensure the unit would meet stringent shocks and vibrations associated with launch, cruise and landing.
But surviving the flight was only the radio's first hurdle. Mars is an inhospitable planet. Anyone who has walked on a rocky surface, or tried getting sand out of clothes is familiar with a couple of Martian hazards. The climate is equally unforgiving. Temperatures can drop from daytime highs of 30¡C to lows of -30·C in a single Mars day (around 24.62 hours long), and often drop to -100·C at night. For protection from the intense Martian cold and sandy terrain, the rover's radio sits inside a Warm Electronics Box (WEB), a feature Strand said was needed to provide "better internal thermal regulation." Although the Rover is solar powered, a backup battery had to be installed in case the solar cells failed. That way the entire primary seven day Rover mission could be completed on battery power alone.
Pearl said knowing that the Rnet only had one chance was unnerving. He admits to at times wondering if DATARADIO had taken on too much. "Like any good conservative engineer, you wonder about what can happen," he said.
As it turns out, only one communication-related problem arose. On the 37th day of the mission, an "energetic" particle from a solar flare hit the rover, and the system's computer reset the modem, temporarily cutting communication to the Lander. "It could have potentially destroyed the circuitry, and that's something that future telecom hardware will have to deal with -- immunity to radiation," Stride said. NASA engineers eventually found a weakness in the computer's configuration, proving that the shutdown had nothing to do with the telecommunication system.
Pearl remembers some tense moments when he first learned of the problem. "There was some apprehension, since no one has tried to crash land on a planet with one of our modems before, and that is literally what they were doing," he said. Pearl is also used to accepting blame. "In our business, that's almost universal. People sit down, hit 'return,' and when nothing happens, they say it must be the modem."
Small glitches aside, Pathfinder's telecommunication's system is still a considerable feat. To remind him of how great a feat, Stride keeps a replica of the telecommunications system from the old Mars Viking mission, a much larger and slower version of the Pathfinder transponder radio. He predicts that in another twenty years the radio system on Pathfinder will seem equally antiquated. But he added that Pathfinder's greatest accomplishment is the kickstart it has given to the American space program. The mission proves that interplanetary exploration and important scientific research can be carried out on a shoestring budget.
All the publicity certainly has not hurt DATARADIO. When customers call inquiring about the RNet's ability, he half-jokingly tells them they are good enough to go to Mars. "Our other modems are mostly found on poles, railways, or in the trunks of police cars. So it gave our people a real thrill," he said.
With a good working relationship already established, Pearl said DATARADIO may be asked to help JPL again. With each mission, however, comes a different challenge.
"They (NASA) have plans for missions that will carry communications from the rover to orbit. They have given us the heads-up." ET